Optical sensors are used in a number of applications ranging from scanning a bar code on a package or digitizing a document for display or printing to optical communications systems. Optical sensors generally operate by detecting electromagnetic energy and producing an electrical signal that corresponds to the intensity of the electromagnetic energy striking the optical sensor. Multiple optical sensors are generally used and are often geometrically positioned in arrays with individual optical sensor corresponding to a respective pixel in the resulting electronic display (the term pixel and optical sensor as used in the art and as used in this application are interchangeable). Such arrays allow a larger spatial area to be scanned than could otherwise be performed by a single optical sensor. Other applications may use raster scan techniques in which fewer optical sensors are needed but an object or spatial area is scanned in an incremental pattern until the object or spatial area is completely scanned.
An electrical signal from each optical sensor is typically conditioned by an output modifier. The output modifier conditions the signal or converts the electrical signal into an output signal that can be easily understood by a computer processor. The function of the output modifier may be performed by a charge to voltage amplifier or an analog to digital (A/D) convertor.
The output signal from the output modifier, corresponding to a respective optical sensor is next processed in a manner consistent with the specific application. In one application, a computer processor may function as a signal processor that assembles the various output signals and displays or prints the resulting picture. In another application, a computer processor could use the output signal to stop a conveyor belt when groceries have been moved up to the check-out register. The applications in which optical sensors can be used is without bound.
An optical sensor array generally comprises multiple photodetectors and an electrical circuit corresponding to each individual photodetector. The photodetector produces an electrical signal in proportion to the electromagnetic energy striking the photodetector. The electrical circuit stores the electrical signal produced by the photodetector. The optical sensor array may also include a timing circuit that provides a timing sequencing for internal and external operation of the optical sensor array. In addition, the optical sensor array may incorporate an output modifier that conditions the electrical signal into a usable form for a signal processor such as a general purpose computer processor.
The photodetector generally detects electromagnetic energy in a specific bandwidth that is optimized for each application. Photodetectors can be manufactured from different materials and by different processes to detect electromagnetic energy in varying parts of the electromagnetic spectrum and over varying bandwidths within the spectrum. In order to provide an optimal amount of electromagnetic energy for the optical sensor to detect, a source of electromagnetic energy is often utilized in the form of a light or laser; however, there is no requirement that a source of electromagnetic energy be incorporated. One of the most often used photodetectors is the photodiode.
Optical sensors may be manufactured in many semiconductor technologies including MOS (Metal Oxide Semiconductors), CMOS (Complementary MOS), I2L, J-FET, or Bi-CMOS. Each of the manufacturing technologies have trade offs with respect to performance, manufacturing cost, and required associated supplies and interface circuits. Optical sensors have previously been manufactured based on CCD (Charge Coupled Device) technology. Generally CCD's require a dedicated process technology, require multiple supplies, require more complicated interface electronics, and have limited capability for integrating other electronic functions and are generally more expensive than the other available technologies.
One type of optical sensor often used in non-CCD technologies includes a passive integrator electrical circuit. In the passive integrator, a photodiode (and its associated junction capacitance and attached parasitic capacitance) are prebiased to a high reverse voltage. The photodiode generates a photocurrent which discharges the capacitance, thereby causing the voltage to decrease. The output voltage for this type of optical sensor is generally non-linear with respect to the integrated charge since the diode capacitance is a function of the diode voltage.
A further disadvantage of the passive integrator is that the integrating capacitance (photodiode and parasitic capacitances) is determined primarily by the photodiode size. Thus sensitivity cannot be increased by increasing the photodiode size since the capacitance will increase approximately proportionally.
A further disadvantage of the passive integrator is that the high reverse voltage during operation will cause dark current to flow even in absence of light. The diode current is given by the following equation: ##EQU1## where: I.sub.s is the saturation current,
q is a constant, PA1 V is the diode voltage, PA1 k is a constant, PA1 T is the temperature in degrees Kelvin, and PA1 I.sub.p is the photodiode current generated by incident light.
For large reverse diode voltage ##EQU2## EQU I=-(I.sub.s +I.sub.p)
The dark current in absence of light is -Is for large reverse diode voltage. This dark current doubles every 6-10 degrees Kelvin. For high sensitivity applications, the dark current can be comparable to the photodiode current for high temperature applications. A high dark current diminishes the usable data that can be obtained from the optical sensor. Also, variations of the dark current between photodiodes in an array due to process variation will cause an output non-uniformity.